Implantable medical devices (IMDs) are known and have been used for therapeutic and functional restoration indications for animals, including humans, for a number of years. These IMDs must be constructed in such a way that produces a hermetically sealed housing or case and of a material that is biocompatible. Ceramics, epoxies, and metals, such as titanium or titanium alloys have been the mainstay for many IMDs, including implantable pulse generators, pacemakers, and drug delivery pumps, for example.
A new class of material, known as thermoplastic liquid crystal polymers (LCP) have a unique combination of properties that make it well suited for encasing IMDs. LCP is extremely inert in biological environments and has barrier properties an order of magnitude greater than epoxy plastic materials and is virtually impermeable to moisture, oxygen, and other gases and liquids. The permeability of LCP to water vapor and to oxygen is close to that of glass. Sealed LCP enclosures have been tested and have passed initial hermeticity helium leak testing, per MIL-STD-883D.
LCP combines the low cost and light weight of a polymer with suitable dielectric properties and protective capabilities. High frequency performance is possible because of the dielectric constant and loss properties of LCP are much lower than those of conventional materials.
LCP can be precision molded and sealed using conventional thermoplastic welding techniques to create a hermetic seal. LCP is also readily processed by injection molding and thermoforming using conventional equipment at fast speeds with excellent replication of mold details, making a wide variety of shapes and configurations now more feasible. LCP packaging may also be laser welded at the bond line using infrared (IR) laser to create the seal. The LCP material is transparent to IR, so the beam passes through the LCP material with minimal absorption. An IR-absorbant material may be added to the LCP at the bond line, localizing heating to the immediate seal area. The welded seal is formed from the LCP material.
Implantable medical devices, such as an implantable pulse generator (IPG), typically include a hermetically sealed titanium case containing a power source and associated circuitry and glass filled conductor feed-thrus to allow the electronic signals generated by the circuitry to interface to a lead and electrode, sensor, and/or antenna, for example. Manufacturing is made more difficult because the glass filled feed-thrus must be welded to the housing in order to maintain a hermetic seal. The feed-thru locations are also restricted by the manufacturing and hermetic sealing requirements. The size of the IMD is generally dominated by the size of the power source and manufacturing limitations, and typically the titanium case ends up being at least a few centimeters in diameter and half a centimeter thick.
Many IMDs also include a rechargeable power source, such as a Litium Ion cell (battery), and a power receive coil for recharging the battery through a Radio Frequency (RF) magnetic field (or the like) generated outside the body. When these IMD's are encased in metal, the titanium acts as a shield, and reflects or absorbs much of the energy intended to recharge the internal battery. The internal power receive coil also needs to be large enough to receive the radio frequency (RF) magnetic field energy through the metal case, which also affects the size of the IMD.
Implant depth of the IMD is also affected by the case material. If it is too deep, the metallic case creates so much shielding that the recharge signal energy is unable to provide adequate recharging. Increasing the frequency of the recharge signal increases the blocking effect of the case which in turn results in an increase in heat produced by the case. On average, the RF magnetic field energy generated externally needs to be ten times greater than the necessary RF magnetic field energy received by the receive coil for adequate recharging, due in large part to the losses created by the titanium case.
IMDs may also incorporate wireless telemetry to provide remote control and programming features. The inclusion of a UHF antenna for the wireless telemetry outside the titanium case is necessary as the shielding offered by the titanium case will severely limit (effectively eliminate) radio wave propagation through the case. The antenna connection will be made through a feed-thru similar to that used for the lead connections. Alternatively, the wireless telemetry signal may be coupled inside the IMD onto a stimulus output channel and coupled to the antenna with passive filtering/coupling elements/methods.
FIGS. 1A and 1B show a typically IMD 20 and associated components. A metallic clam shell type housing (i.e., case) having a top half 22 and a bottom half 23, is provided to house the circuitry 24 and power source 26. One or more feed-thrus 28 are welded to the top half 22 and/or bottom half 23 (see FIG. 1B). A header 30 is typically coupled to the housing 22, 23, and feed-thrus 28 to provide an electrical connection point for the components not in the metallic housing (e.g., leads 32, electrodes 34, sensors 36, and/or antenna 38). The need to weld the feed-thrus 28 to the housing 22, 23 and couple the header 30 to the housing are both difficult manufacturing steps, and both create failure points, not to mention added expense to the manufacturing process. The IMD 20 is then subjected to a vacuum bake-out and then backfilled with an inert gas and the housing top half 22 and bottom half 23 are welded to produce a hermetically sealed device.
There remains a need for an improved IMD that can be constructed in a case (i.e., encased) that offers the liquid impermeability features of conventional materials, but also includes improved features, such as simpler manufacturing and less unwanted shielding, which will allow for a smaller and less expensive IMD that can be physically located in a wider range of areas within the body.